Tracing paper has in most cases been used as transfer paper in electrophotographic copiers operated by the xerographic system. However, tracing paper does not have satisfactory dimensional stability, keeping quality, mechanical strength and water resistance. When these properties are required, matted films (tracing film) made of polyethylene terephthalate resin (hereunder referred to as PET resin) or cellulose triacetate resin (hereunder referred to as CTA resin) have been used as transfer film. To achieve good fixation between toner and transfer film, the thermal fixation assembly of the electrophotographic copier must be kept at a high temperature. When the heating mechanism of that assembly is of radiant-heating type, it is particularly important that the assembly is kept at a temperature of at least 250.degree. C. or even higher than 300.degree. C. If the PET film or CTA film whose heat resistance is not adequate is used as transfer film, they undergo significant rippling (i.e., formation of waves on the surface) during thermal fixation that considerably impairs their flatness and at the same time, shrinks them to a great extent. If the temperature of the thermal fixation is not high enough to cause rippling of the film, the toner is fixed to the film so weakly that it easily separates from the film. Furthermore, the conventional heat resistant transfer film does not have satisfactory writing quality or adequate fixation to toner. "Writing quality" refers to the ability of a transfer film to be written on with pencil, ink, or the like; this is an important property for a transfer film, in that it is often desirable to modify an image on a transfer film by ordinary writing thereon. As a further disadvantage, the conventional transfer film is too transparent to be easily detected when it gets stuck in the electrophotographic copier. Most electrophotographic copiers use the principle of light shielding to detect a stuck paper or film in the machine, but a transparent film is not detected if it gets stuck and hence the machine does not stop.
I previously made various studies on an improved transfer film having none of the problems described above, that is, a transfer film for an electrophotographic copier that can be used in an electrophotographic copier using a radiant-heating or other high temperature type of thermal fixation assembly without undergoing rippling to impair the film flatness and which has an extremely small heat shrinkage ratio, has good fixation to toner and which has improved writing quality. As a result, I accomplished an invention for which patent was applied as Japanese patent application No. 44028/79 (corresponding to U.S. patent application Ser. No. 140,400, filed Apr. 14, 1980, now U.S. Pat. No. 4,352,847). According to one aspect of that invention, a transfer film made of heat resistant plastics having a UL (Underwriters Laboratories) temperature index of 120.degree. C. or more retained high film flatness without undergoing rippling, had a extremely small heat shrinkage ratio and achieved good fixation to the toner even if it was processed by an electrophotographic copier of the type that maintained the thermal fixation assembly at an elevated temperature. According to another aspect of that invention, a film at least one side of which was matted to a surface roughness of 1.0 micron or more (1) had improved fixation to toner because of fused toner adhered to both projections and recesses in the surface and (2) that film had writing quality and could be used not only as an intermediate but also as ordinary writing paper. According to still another aspect of that invention, a film having an opacity (as measured according to the method specified in JIS P8138-1976) of from 20 to 65% had the advantage of (2) and yet could be detected as easily as ordinary paper when it got stuck in the electrophotographic copier of today, such as Xerox 3600, 2400, 9200 and 7200 which use the principle of light shielding to detect a stuck paper. However, such a method does not stop the machine even if transparent paper or a transparent film gets stuck in the machine, and the operator does not know where the film is. In contrast, the transfer film having an opacity of 20 to 65% can be easily detected if it is stuck and the operator can readily find its location. The specification of Japanese patent application No. 44028/79 mentions a polysulfone resin as an example of the plastics having the properties described above.
Even the transfer film for electrophotographic copier proposed in Japanese patent application No. 44028/79 has several problems. If its surface resistance is high, it does not run (pass) through the photographic copier smoothly and is easily stuck. In addition, the toner particles pop (i.e., explode) over the film and as a result, a sharp image is not produced. If the film has a high friction coefficient, two layers of the film easily cohere and run through the machine simultaneously or they won't run (pass) through the machine smoothly.